Adds serverId to the globalId part of a Xid

This to make it impossible that two instances may produce the same random
seed for its Xid generator and happen to generate the same ID for
different transactions.

Also uses the default constructor of Random() because it's a more random
seed than just current time.
Merge pull request #1571 from tinwelint/1.9-xid-with-serverid

Adds serverId to the globalId part of a Xid